1. Field of the Invention
The present invention relates to flow measuring apparatus. More particularly, it relates to sensors for vortex flowmeters and to piezoresistive pressure sensors.
2. Description of the Prior Art
It has been known for many years that vortices are developed in a fluid flowing past a non-streamlined obstruction. It also has been known that with certain arrangements vortices are developed by alternately shedding at regular intervals from opposite edges of the obstruction to form corresponding rows of vortices. Such vortices establish a so-called von Karman "vortex street," which is a stable vortex formation consisting of two nearly-parallel rows of evenly-spaced vortices travelling with the flow stream.
In a yon Karman vortex street, the vortices of one row are staggered relative to those of the other row by approximately one-half the distance between consecutive vortices in the same row. The spacing between successive vortices in each row is very nearly constant over a range of flow rates, so that the frequency of vortex formation is correspondingly proportional to the velocity of the fluid. Thus, by sensing the frequency of vortex formation it is possible to measure the fluid flow rate. Devices for that purpose are often referred to as vortex meters.
Various types of vortex meters have been available commercially for a number of years. Typically, these vortex meters comprise a vortex-shedding body mounted in a flow tube together with a sensor for detecting the frequency of vortex formation. Sensors used to detect the vortices often include diaphragms which fluctuate in response to alternating differential pressure variations generated by the vortices. For example, in U.S. Pat. Nos. 4,085,614 to Curran et al. and in 4,520,678 to Koziol et al., pressure applied to the diaphragms is transferred to a piezoelectric sensor which then produces electronic signals responsive to differential pressure applied to the diaphragms. This differential pressure measurement is used, in turn, to measure the frequency of vortex formation and ultimately the fluid flow rate or velocity.
A limitation of this type of sensor is that it is capable of making only one measurement in a single process penetration, specifically, measuring the frequency of differential pressure fluctuations used to compute the flow velocity. Additional instruments and process penetrations would be required in order to obtain additional measurement quantities, such as process fluid pressure or temperature. This increases the risk of releases of fugitive emissions and fluid loss and bears an increased cost for the purchase and installation of the additional instruments. Another disadvantage of additional process penetrations is the loss of accuracy in the measurements due to varying sampling points. Since the physical characteristics of the fluid changes within the flow, accurate measurements would require a common source point from which to sample within a single penetration.
It is the object of the present invention to provide a sensor which provides multimeasurement capabilities at a common source point within a single process penetration for use, in particular, within a vortex flowmeter.
A more specific object of the invention is to provide a more accurate computation of fluid density by providing a means for multimeasurement capabilities. The frequency and amplitude of the alternating differential pressure signals created by the shedding vortices, the process fluid pressure, and temperature can all be measured at a common source point with a single sensor and in a single process penetration.
A further specific object of the invention is to employ the use of a sensor with piezoresistive materials which is capable of making multiple measurements.
A further specific object of the invention is to use the multiple measurements to derive other computations, such as density, absolute and kinematic viscosity, Reynold's No., and the mass flow rates of both liquids and gases.
Other general and specific objects of this invention will be apparent and evident from the accompanying drawings and the following description.